Vehicle transmissions transfer power and torque from an engine to the vehicle output (axles or wheels). When the vehicle is equipped with an additional primary mover, such as an electric machine, the transmission may also transfer torque and power from the electric machine to provide traction for the vehicle. Fixed-gear transmissions allow the engine to operate within a narrow range of speeds while providing a wide range of output speeds.
Different gears or modes of the transmission provide different, selectable speed ratios or gear ratios, and also different levels of torque multiplication. Changing the speed ratio of the transmission changes the ratio of input speed to the output speed and changes the torque multiplication between the input and the output. Hybrid powertrains include both an internal combustion engine and an alternative power source, such as one or more electric machines coupled with an energy storage device.